The reaction catalyzed by the converting enzyme was first described only a few short years ago. Since the converting enzyme is necessary for the formation of the physiologically active species, angiotensin II, it is of paramount importance to have an understanding of the biochemistry and physiology of this enzyme. It is planned to study its distribution in several different organ systems and species under normal circumstances as well as abnormal ones. We have adopted the biological assay (aorta, rat colon), the radioimmunoassay, and the fluorometric method for the quantitative determination of this enzyme. Evidence has accumulated which indicates that the lung is the principal site of conversion of the inactive decapeptide angiotensin I to the vasoactive species angiotensin II. With subcellular fraction and density gradient techniques the enzyme seems to be localized on the plasma membrane in pulmonary tissue. We will use the fraction displaying highest converting enzyme activity as a starting point for obtaining an enzyme of sufficient purity for kinetic studies. It is planned to study the effect of various ions, sulfhydryl reagents, and other factors on enzyme activity. Particular emphasis will be placed on potential inhibitors. Other peptides, bradykinin, bradykinin derivatives, angiotensin derivatives, BPF (bradykinin potentiating factor) will be used to gain an insight into the mechanism of the converting enzyme. In addition, our studies are also designed to clarify the relationship between converting enzyme and bradykininase. It is possible that the level and activity of this enzyme in lung and other tissue may be altered in cardiovascular disease states and thus an understanding of the biochemistry of the converting enzyme will aid in an understanding of those diseases affecting the cardiovascular system.